A pipe string being assembled in a wellbore is supported at or near the upper end by a spider that rest on the drilling rig floor or by an elevator that is suspended from the rig traveling block above the pipe string. As the wellbore becomes deeper, additional sections of pipe are added to the pipe string. Conversely, removal of the pipe string from the wellbore requires the removal of pipe section from the string. To facilitate adding and subtracting pipe sections from the pipe string the spider engages with the pipe string to prevent it from falling into the wellbore.
To support the weight of the pipe string while additional pipe sections are added to the pipe string, a plurality of slips extending downward from the spider are engaged inward to compress against the surface of the pipe. These slips have gripping surfaces called dies that contact and hold the outer surface of the pipe. A plurality of gripping dies, usually three or more, are vertically distributed on each slip.
The weight of the pipe string on the spider slips increases the inward force applied by the slips and gripping dies against the pipe face. The gripping dies often receive substantial torque transmitted from the spider to the pipe. The torque is often collateral with other rig floor activities. Gripping dies that have teeth on a cylindrical surface that approximates the pipe outer cylindrical surface tilt somewhat as a result of machine slack and strain when torque is being, transmitted to the pipe. When the dies are tilted, one edge of the dies tends to gouge into the pipe. The resulting load concentration tends to distort the pipe with the unintended consequence of pipe surface damage. This pipe surface damage can result in fatigue and failure of the pipe over extended periods of use.
Curved dies are typically shaped to correspond with a specific diameter pipe so as to engage with a maximum area of a pipe. Because of this, the gripping dies of the spider must frequently be exchanged with gripping dies having a different curvature when the need for switching between pipes of different sizes arises.
To increase versatility of the die and simultaneously minimize gouging of the pipe, a V-shaped gripping die as disclosed in U.S. Pat. No. 7,775,270 to D. Sipos can be used in lieu of traditional curved dies. Such a V-shaped die is composed of two wide flat surfaces that combine at an apex to form a V-shaped flat contact surface that engages the pipe along two vertical lines away from the outer edges of the die. Because V-shaped gripping dies do not make contact with the pipe with an edge, loads are more evenly distributed over the die surface. This serves to reduce gouging of the pipe during use and while accommodating use with a variety of pipe diameters. However, as the weight of the pipe string increase the two vertical lines of contact between die and pipe surface may concentrate the gripping force on the pipe causing an unintended increase in the incidence of markings on the pipe surface. These markings may weaken the pipe and, over the course of use increase the risk of damage or failure of the pipe.